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NASA scientists have developed a new climate model that indicates that the most violent severe storms and tornadoes may become more common as Earth’s climate warms.

Image right: Screenshot from an animation showing clouds over North America on August 2, 2000, as measured by GOES-11. Credit: NASA/NOAA.+ Download movie (.mpg)

Previous climate model studies have shown that heavy rainstorms will be more common in a warmer climate, but few global models have attempted to simulate the strength of updrafts in these storms. The model developed at NASA’s Goddard Institute for Space Studies by researchers Tony Del Genio, Mao-Sung Yao, and Jeff Jonas is the first to successfully simulate the observed difference in strength between land and ocean storms and is the first to estimate how the strength will change in a warming climate, including “severe thunderstorms” that also occur with significant wind shear and produce damaging winds at the ground. This information can be derived from the temperatures and humidities predicted by a climate computer model, according to the new study published on August 17 in the American Geophysical Union’s Geophysical Research Letters. It predicts that in a warmer climate, stronger and more severe storms can be expected, but with fewer storms overall.

Global computer models represent weather and climate over regions several hundred miles wide. The models do not directly simulate thunderstorms and lightning. Instead, they evaluate when conditions are conducive to the outbreak of storms of varying strengths. This model first was tested against current climate conditions. It was found to represent major known global storm features including the prevalence of lightning over tropical continents such as Africa and, to a lesser extent, the Amazon Basin, and the near absence of lightning in oceanic storms.

The model then was applied to a hypothetical future climate with double the current carbon dioxide level and a surface that is an average of 5 degrees Fahrenheit warmer than the current climate. The study found that continents warm more than oceans and that the altitude at which lightning forms rises to a level where the storms are usually more vigorous.

These effects combine to cause more of the continental storms that form in the warmer climate to resemble the strongest storms we currently experience.

Lightning produced by strong storms often ignites wildfires in dry areas. Researchers have predicted that some regions would have less humid air in a warmer climate and be more prone to wildfires as a result. However, drier conditions produce fewer storms. "These findings may seem to imply that fewer storms in the future will be good news for disastrous western U.S. wildfires," said Tony Del Genio, lead author of the study and a scientist at NASA’s Goddard Institute for Space Studies, New York. "But drier conditions near the ground combined with higher lightning flash rates per storm may end up intensifying wildfire damage instead."

The central and eastern areas of the United States are especially prone to severe storms and thunderstorms that arise when strong updrafts combine with horizontal winds that become stronger at higher altitudes. This combination produces damaging horizontal and vertical winds and is a major source of weather-related casualties. In the warmer climate simulation there is a small class of the most extreme storms with both strong updrafts and strong horizontal winds at higher levels that occur more often, and thus the model suggests that the most violent severe storms and tornadoes may become more common with warming.

The prediction of stronger continental storms and more lightning in a warmer climate is a natural consequence of the tendency of land surfaces to warm more than oceans and for the freezing level to rise with warming to an altitude where lightning-producing updrafts are stronger. These features of global warming are common to all models, but this is the first climate model to explore the ramifications of the warming for thunderstorms.

The Goddard Institute for Space Studies is a leading center in the study of Earth’s past, present and future climates.